From: Rajesh Venkatasubramanian <vrajesh@umich.edu>

This patch adds prefetches for walking a vm_set.list.  Adding prefetches
for prio tree traversals is tricky and may lead to cache trashing.  So this
patch just adds prefetches only when walking a vm_set.list.

I haven't done any benchmarks to show that this patch improves performance.
 However, this patch should help to improve performance when vm_set.lists
are long, e.g., libc.  Since we only prefetch vmas that are guaranteed to
be used in the near future, this patch should not result in cache trashing,
theoretically.

I didn't add any NULL checks before prefetching because prefetch.h clearly
says prefetch(0) is okay.